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Old 03-12-2013, 08:34 PM   #1 (permalink)
 
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Default Electropolishing Stainless steel, Aluminium, Titanium, and copper alloys

I know we have a few turners amongst us who turn occasionally in stainless steel, and i thought i would share a process with you.

By electropolishing a piece of stainless, we in effect dissolve the entire surface area of the piece but only by a micron or too so we are not to Jeopardize any of our dimensional tolerances. because of this, all those tony blemishes and faint scratches dissapear giving you a finish with a gleam like its coated in water.

So, how does it work?



Well very much in the same way as anodising. The same equipment and setup is used, with the workpiece being connected to the anode. only this time rather than gowing a microscopic layer of crystals over its surface like aluminium does, its outer layer begins to break down atom by atom literally just skimming a layer off.

you need an anodising tank, and i would recommend any LDPE plastic container. i use one of those household plastic storage containers with the blue clip on lid and its great picked up at my local diy store. keep away from glass fish tanks please.. tanks of acid and glass isnt a good idea. and even if you make a plastic tank, spend $20 on a acid spill kit. i can assure you its one of those things you never realise the importance of until you spill a load of acid everywhere... on my occasion it was a concrete floor luckily not indoors. only took a few secondsfor the concrete to start smoking. Cleaned it well though

for your cathodes, you can use lead flashing, or just a bar of aluminium 6061T6 alloy or similar which is the industry standard.

your electrolyte is made in a stronger form than what is used in anodising. this is the solution which will conduct electricity and of course aid dissolving the part.

Place the water in the tank first, and then pour the acid's into the tank carefully and slowly one by one.. you dont want this splashing everywhere.

Here are recipes for the type i think we would use:






Wear eye protection and a lab coat while doing this as it makes a mess of clothes

ALWAYS add acid, ... Dont put the acid in first then pour all the water onto it causing it to splash everywhere and flashboil. A violent reaction occurs when it is done in this manner causing the temperature of it to flash up which can make it spit more.



with your Cathodes in and wired together. connect this to the negative of your power supply. your workpiece, becomes the anode by having the postive lead attached to it. lower it and suspend it in the middle of the tank taking care that your part is clear from the sides and wont short out by touching a cathode.

Voltage and mA to set your power supply on can be found above on the charts. You should start seeing a nice difference after 10 minutes, its up to you how long you let it run for. i suggest you do some experimentation with a scrap piece and a caliper.

Lots of gases are produced doing this too albeit most of it hydrogen. setup some good ventilation to get those corrosive fumes out of your shop and lungs.

once you are finished, turn off your power supply and remove your piece wearing your chemical rated gloves and give the part a good rinse down.

Then, admire your shine



after electropolishing, rinse your part in dilute nitric acid which will dissolve the sulfates and other residues left on the surface from the polishing process. if these are left on over time they will crumble to white powder ruining the look of your shine. Dont forgetto wash in water after any process step

Last edited by BradG; 03-12-2013 at 09:21 PM.
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Old 03-12-2013, 08:42 PM   #2 (permalink)
 
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MAN you have way to much time on your hands. I only wish.

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Old 03-12-2013, 08:49 PM   #3 (permalink)
 
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That tutorial really makes me want to give this a whirl.

I make biodiesel so I'm a little familiar with the chemicals and the safe handling of them.
Thank you very much for sharing this kind of information.
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Old 03-12-2013, 08:51 PM   #4 (permalink)
 
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no problem

Be sure to drop by www.penchemistry.co.uk Theres a few other tutorials on there which may catch your eye
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Old 03-12-2013, 10:24 PM   #5 (permalink)
 
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Crazy stuff. Thank you. Way smart you are.
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Old 03-13-2013, 02:56 PM   #6 (permalink)
 
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I like the idea of anodizing and electropolishing, just not sure if I want to keep those kinds of acids in the shop. Is there any other ways to harden aluminum or get stainless to that level of shine? I have a few days off this week and bought some stainless bolts and washers and was going to try a thing or two.
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Old 03-13-2013, 03:19 PM   #7 (permalink)
 
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Dunno about the other softer metals, but I find that wet 'n dry up to 2000 grit combined with some elbow grease works fine for stainless.
But I do have forearms like Popeye!!! :)
Are you talking about the light surface scratches that you always seem to get with polished ally, or deeper scratches?
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Old 03-13-2013, 03:23 PM   #8 (permalink)
 
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Not sure yet, I picked up 2 bolts and a few washers and am going to get started over the weekend.
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Old 03-13-2013, 03:35 PM   #9 (permalink)
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You dont collect the hydrogen gas for other fun things to have show and tell with?
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Old 03-13-2013, 04:03 PM   #10 (permalink)
 
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Quote:
Originally Posted by Displaced Canadian View Post
I like the idea of anodizing and electropolishing, just not sure if I want to keep those kinds of acids in the shop. Is there any other ways to harden aluminum or get stainless to that level of shine? I have a few days off this week and bought some stainless bolts and washers and was going to try a thing or two.
To be honest so long as you have a plastic container with a sealable lid you dont have much to worry about. anodising baths are only dilute to around 15-20% sulphuric which is only half as strong as what is in your car battery.
I agree, the other chems for electropolishing and plating are a little more toxic though.

With regards to hardening, you could look into precipitation or age hardening. Here is an article i have on it. not sure where i picked it up from, but never the less its a good reference about the process.

************************************************** ****

Precipitation hardening, or age hardening, provides one of the most widely used mechanisms for the strengthening of metal alloys. The fundamental understanding and basis for this technique was established in early work at the U. S. Bureau of Standards on Duralumin.

The importance of theoretical suggestion for the development of new alloys is clear from the historical record. At the end of the 19th century, cast iron was the only important commercial alloy not already known to western technology at the time of the Romans. When age hardening of aluminum was discovered accidentally by Wilm, during the years 1903 -1911, it quickly became an important commercial alloy under the trade name Duralumin.
The strength and hardness of some metal alloys may be enhanced by the formation of extremely small uniformly dispersed second-phase particles within the original phase matrix in a process known as precipitation or age hardening. The precipitate particles act as obstacles to dislocation movement and thereby strengthen the heat-treated alloys. Many aluminum based alloys, copper-tin, certain steels, nickel based super-alloys and titanium alloys can be strengthened by age hardening processes.
In order for an alloy system to be able to be precipitation-strengthened, there must be a terminal solid solution that has a decreasing solid solubility as the temperature decreases. The Al-Cu (Duralumin is an aluminum alloy of 2XXX group) phase diagram shown in Figure 1 shows this type of decrease along the solvus between the α and α+θ regions. Consider a 96wt%Al 4wt%Cu alloy which is chosen since there is a large degrease in the solid solubility of solid solution α in decreasing the temperature from 550C to 75C.

Figure 1: The aluminum rich end of the Al-Cu phase diagram showing the three steps in the age-hardening heat treatment and the microstructures that are produced.

In an attempt to understand the dramatic strengthening of this alloy, Paul D. Merica and his coworkers studied both the effect of various heat treatments on the hardness of the alloy and the influence of chemical composition on the hardness. Among the most significant of their findings was the observation that the solubility of CuAl2 in aluminum increased with increasing temperature.
Although the specific phases responsible for the hardening turned out to be too small to be observed directly, optical examination of the microstructures provided an identification of several of the other phases that were present. The authors proceeded to develop an insightful explanation for the hardening behavior of Duralumin which rapidly became the model on which innumerable modern high-strength alloys have been developed.
They summarized the four principal features of the original Duralumin theory:
  • age-hardening is possible because of the solubility-temperature relation of the hardening constituent in aluminum,
  • the hardening constituent is CuAl2,
  • hardening is caused by precipitation of the constituent in some form other than that of atomic dispersion, and probably in fine molecular, colloidal or crystalline form, and
  • the hardening effect of CuAl2 in aluminum was deemed to be related to its particle size.

The precipitation-hardening process involves three basic steps:
1) Solution Treatment, or Solutionizing, is the first step in the precipitation-hardening process where the alloy is heated above the solvus temperature and soaked there until a homogeneous solid solution (α) is produced. The θ precipitates are dissolved in this step and any segregation present in the original alloy is reduced.
2) Quenching is the second step where the solid α is rapidly cooled forming a supersaturated solid solution of αSS which contains excess copper and is not an equilibrium structure. The atoms do not have time to diffuse to potential nucleation sites and thus θ precipitates do not form.
3) Aging is the third step where the supersaturated α, αSS, is heated below the solvus temperature to produce a finely dispersed precipitate. Atoms diffuse only short distances at this aging temperature. Because the supersaturated α is not stable, the extra copper atoms diffuse to numerous nucleation sites and precipitates grow. The formation of a finely dispersed precipitate in the alloy is the objective of the precipitation-hardening process. The fine precipitates in the alloy impede dislocation movement by forcing the dislocations to either cut through the precipitated particles or go around them. By restricting dislocation movement during deformation, the alloy is strengthened.
Age Hardening Precipitation. The strongest aluminum alloys (2xxx, 6xxx and 7xxx) are produced by age hardening. A fine dispersion of precipitates can be formed by appropriate heat treatment.
A general model for decomposition is given, followed by details of the precipitation sequences in 4 specific alloy systems: Al-Cu, Al-Cu-Mg, Al-Mg-Si and Al-Zn-Mg. The Al-Cu system is used as the main example of decomposition, i.e.
a0 (SSSS) → GP zones → θ'' → →θ' → θ or, more fully:
a0 (SSSS) → α1 + GP zones → α2 + θ'' → α3 + θ' → α4 + θ
Age Hardening Strengthening. The 3 main mechanisms are:
  • Coherency strain hardening;
  • Chemical hardening;
  • Dispersion hardening

Coherency strain hardening results from the interaction between dislocations and the strain fields surrounding GP zones and/or coherent precipitates. Chemical hardening results from the increase in applied stress required for a dislocation to cut through a coherent (or semi-coherent) precipitate. This in turn depends on a number of factors, including:
  • the extra interfacial area - and hence energy - between precipitate and matrix;
  • the possible creation of an anti-phase boundary (APB) within an ordered precipitate and
  • the change in separation distance between dissociated dislocations due to different stacking fault energies of matrix and precipitate.

Dispersion hardening occurs in alloys containing incoherent precipitates or particles - i.e. typically those that have been overaged. This hardening results from the increased shear stress required for dislocations to by-pass these obstacles.
As mentioned above, the precipitation reactions in Al-Cu are quite complex. The equilibrium phase CuAl2 is difficult to nucleate so its formation is preceded by a series of metastable precipitates. Guinier and Preston first discovered many of the age hardening phenomena. The first two precipitates to form in the sequence are, therefore, known as GP zones. GP1 consists of 10 nm diameter copper-rich plates on {100}Al planes. These develop into GP2 zones which are also coherent plates 10 nm thick and 150 nm diameter. These lead to maximum hardening. Theta' /θ'/ precipitates then replace the GP zones as semi-coherent particles, a stage known as over-aging because the hardness begins to decrease. The equilibrium phase CuAl2 has a tetragonal crystal structure and contributes little to hardness.
In the field of 6000 series precipitation hardening aluminum alloys, for instance, process models have been able to describe the effect of quench-induced precipitation on structural defects on the hardening potential during isothermal low-temperature aging.
The fracture toughness of 7000 series alloys has been related to some elements of the microstructure resulting from the thermo-mechanical treatment in phenomenological models. The general strategy of process modeling is to use individual equations which have been developed for well defined experiments and try to integrate them in an integrated manner for the more complex practical situations where coupled effects operate.
However, a good description is still lacking when several of these phenomena are simultaneously operative. The understanding of competitive precipitation of several phases (metastable and stable) on several nucleation sites (e.g. homogeneous and on structural defects) is very limited, as well as the understanding of the shearing/by-passing transition leading to the maximum strength for precipitation hardening materials. The strain hardening behavior of materials containing precipitates (and thus necessarily a solid solution) is poorly understood, and predicting the fracture toughness in cases where several fracture modes are simultaneously operating is not possible in the present state of the art.

**************************************

As Skippys quite rightly said, theres always good old fashioned elbow grease and polishing wheels

Quote:
Originally Posted by skiprat View Post
Dunno about the other softer metals, but I find that wet 'n dry up to 2000 grit combined with some elbow grease works fine for stainless.
But I do have forearms like Popeye!!! :)
Are you talking about the light surface scratches that you always seem to get with polished ally, or deeper scratches?
Steve if you think what you are doing with sand paper, you start with a course grit and work your way up to 1500, 2500 right up to around 12000 if you want to, each time making a finer and finer set of scratches removing the larger ones from before. With electropolishing its dissolving all the scratch marks so you could finish your work off with a 400 grit then electroform and it will look better than it would have been if you sat there and went through all the grits.... as even the finest of scratches the micromesh is leaving, there just isnt any when you electropolish.

Though i would like to put it to the test.. would you be up for polishing a stainless nut, and one unpolished up for my to electropolish? it would be good to see them side by side and see if its really that big of a deal how you do it

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Originally Posted by mredburn View Post
You dont collect the hydrogen gas for other fun things to have show and tell with?
no my wife banned me from most things flammable and explosive some time ago
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